Chapter 7 Module Assembly and Optoelectronic Packaging
7.2 Surface Mount Technology
SMT is an electronic assembly technology that uses an automatic machine to assemble surface mount components onto a printed circuit board or other substrates directly. The components are usually discrete, small, with short leads or leadless. SMT is the critical technology for assembling discrete components into modules or devices.[1−6]
126 Chapter 7 Module Assembly and Optoelectronic Packaging Unlike traditional THT, holes are not required in PCBs to solder components to the designated surface area. The SMT comprises the dispensing, screen printing, attachment, reflow, cleaning, and online testing processes. In brief, it uses tools to dispense glue or paste on the metal traces of a substrate, attaches surface mount devices (SMDs) to the correct locations, and builds electrical and mechanical connections after the reflow, as shown in Figure 7.4.
Figure 7.4 A typical SMT assembly
SMT is the driving force in electronic devices, miniaturization, cost reduction, and in-creasing reliability, and is the milestone for the information technology industry. SMT encompasses SMD, attachment technology, and pick-and-place technologies. The high pack-ing density of SMT enables the electronic product or system to achieve 40%–60% reduction in volume, a 60%–80% reduction in weight, and a 30%–50% reduction in cost. Along with excellent reliability and high-frequency performance of SMD, the SMT process and equip-ment selection and configuration are vital for the quality assurance of electronic products and systems.
Almost all of the electronic systems, telecommunications and computer networks in partic-ular, have adopted SMT technologies. The world production of SMDs grows each year, and OEM and electronic manufacture service (EMS) have become mainstream in the electronics industry. Annual production of traditional DIPs, through-hole resistors, and capacitors are decreasing. SMT will dominate the market in the years to come.
Surface mount technologies involve two areas: one is the material, which includes compo-nents and its manufacture technologies along with supporting materials such as flux, epoxy, and solder; the other is assembly processes, which include screen printing, pick-and-place, reflow, cleaning, inspection, etc.
7.2.1 Features
The basic difference between SMT and THT lies in the operation of “placement” and
“insertion,” which determines many aspects of SMD, such as its packaging format, processes, equipment, and functional differences. Surface mount placement methods can be classified as single-side hybrid, double-side surface mount, or all surface mount. The features of SMT include the following:
(1) high component count and high quality requirement for SMD;
(2) high accuracy assembly and high yield;
(3) complicated assembly steps;
(4) highly automated, requiring automatic equipments;
(5) requiring technology and know-how.
1. The Benefits of SMT
(1) Since there is no through hole and wiring is via buried layers, there are more layout spaces and higher wiring density. With the same put in functionality, reducing the layer count lowers the overall cost.
7.2 Surface Mount Technology 127 (2) It has lower weight, which is suitable for highly mobile and light electronics, such as aviation, space, portable electronics, etc. Lighter weight also contributes to higher physical properties such as resistance to vibration.
(3) Its assembly speed is faster than through-hole, and it is easy to automate, up to 50 thousand parts per hour. This raises productivity and lowers assembly cost.
(4) New solder paste and reflow technologies have promoted the soldering quality and avoided short, cold soldering and deformation problems.
(5) It has a smaller footprint and shorter wiring; lower parasitic inductance and capaci-tance enable higher signal transmission speed, reduced noise, etc.
2. The Disadvantages of SMT
(1) SMT poses increased testing difficulty and costs more because of higher packaging density.
(2) SMT requires adaptation to traditional through-hole components to SMD types, such as chip resistor, chip capacitor, surface mountable quartz, transformer, switch and relay, etc.
7.2.2 Basic Assembly Processes and Work Flow 1. Assembly Method
There are three major SMT assemblies: single-side hybrid, double-side surface mount, and all surface mount. With minor differences in assembly methodology and processes, they share the following common work flow: inspection→screen printing of paste→pick and place→reflow→cleaning→inspection→rework.
The main processes are outlined in Figure 7.5.
Side A first:
Side B:
After insert through-hole devices, go through wave soldring:
Print solder paste
Dispense glue
Place components
Place components
Reflow
Heat cure Flip
Flip
Clean Wave soldering
Insert through-hole devices
Figure 7.5 Hybrid assembly processes
2. Basic processes
(1) During screen printing the solder paste or glue is printed on the PCB and prepared for the soldering. The required equipment is a screen printer, which is an advanced machine in SMT production.
(2) During dispensing the glue is dispensed onto designated locations in order to fix components on the PCB. The required equipment for dispensing is a dispenser, which is normally in the front end of SMT production or right after inspection.
(3) During pick and place the SMD is picked up and placed on the correct location on
128 Chapter 7 Module Assembly and Optoelectronic Packaging the PCB. The required equipment is a “pick and place” machine, which is located after the screen printer.
(4) During curing the glue is cured and holds components onto the PCB. The required equipment is a curing oven, which is located after “pick and place” machine.
(5) During reflow soldering the solder is melted and attachs the SMDs onto the PCB. The equipment required is a reflow oven, which is used after the “pick and place” machine in the SMT line.
(6) During cleaning harmful fluxes and residues are removed from the finished PCB. The required equipment is a cleaner. It can be online or offline.
(7) During inspection the soldering and assembly quality of finished PCB products is checkal. The required equipment for detection is magnifying glasses, microscopes, in circuit testers (ICTs), flying probe testers, automated optical inspections (AOIs), X-ray inspection systems, functional testers, etc. According to inspection requirements, its location in a production line is flexible.
(8) During rework the failed PCB product is reworked. Rework tools include a soldering iron, rework stations, etc. It can be in any appropriate location of the production line.
The four major pieces of equipment for the SMT process are screen printer, “pick and place” machine, AOI, and reflow oven.
Currently, the most popular assembly processes are all SMT and hybrid assembly, which has high integration density and uses both SMDs and through-hole devices. To balance the quality and cost, the reflow and wave soldering processes are the key factors.
7.2.3 Material and Cleaning Processes
Common materials for SMT are adhesive, solder paste, flux, cleaning agent, and anti-oxidation elements. The cleaning process is the basic process in SMT. Without proper cleaning, the flux residue will corrode the PCB. Common cleaning agents include CFC-113 (trifluoromethyl trichloroethane) and methyl chloroform. In addition, stabilizers, such as ethosome, acrylate, or epoxy type compounds, should be added during usage.
The basic requirements of cleaning agents are high degreasing ability, high dissolvability toward colophony and lipid, noncorrosive to metal, do not dissolve high molecular weight compounds, low surface tension, good wetting ability, can be removed from PCB in room temperature, low toxicity, nonexplosive, nonflammable, harmless to humans, stable, and does not react during the cleaning.
Common cleaning methods include soaking, ultrasonic cleaning, air cleaning, and spraying.
In recent years, the development of no-clean solder makes the no-cleaning process popular in SMT, and here are the reasons:
(1) The waste water produced from the cleaning process contaminates rivers and soil, and harms plants and animals.
(2) Solvents that contain chlorofluorocarbons compounds pollute the air.
(3) The residues from solvents are corrosive and lower product quality.
(4) No-cleaning reduces process time and saves money.
(5) The process eliminates the possiblity of damages in the cleaning steps.
(6) The electrical performance of residual flux has been improved and will not affect the product quality.
(7) The no-cleaning process has passed numerous international safety standards, and the no-cleaning flux residues are proven to be stable and noncorrosive.
7.2.4 Surface Mount Components
Surface mount components, abbreviated as SMC or SMD, can be of rectangular,
cylin-7.2 Surface Mount Technology 129 drical, cubic, or irregular shapes. SMD includes packaged semiconductor devices and bare chips. The soldering points or leads of SMD are in the same plane so they can be surface mounted on a PCB. SMD has undergone tremendous growth. Miniaturization is its main trend—it has moved from 1206, 0805, 0603 to 0402 or 0201 foot prints.
Surface-mount components have the following characteristics:
(1) They are small in size, lightweight, and can be mounted on both sides of the substrate for high-density package. For example, the size of a traditional transistor transceiver was reduced to less than 5 mm after adopting SMT.
(2) They have short leads or are leadless, reduced parasitic capacitance and inductance, and improved the high frequency performance.
(3) With their compact structure, SMT products have better vibration and shock resis-tance and higher reliability.
(4) The standardized shapes and footprints, the automatic assembly, the reflow process, the speed and high quality of production, the ease of adaptation for mass production and online inspection all contributed to lower overall cost of ownership.
Common surface mount components (SMCs) includes chip resistors, thick-film resistors, network resistors, film resistors wire wound resistors, thermistors, pressure sensitive resistors, tantalum chip capacitors, film capacitors, chip inductors, chip filters, chip oscillators, chip delay lines, chip switches, and relays. the SMD in the form of packaged semiconductor chips includes small outline transistor (SOT), small outline package (SOP), flat pack (FP), plastic leaded chip carrier (PLCC), quad flat pack (QFP), ceramic packaged device, leadless chip carrier (LCCC), leaded ceramic chip carrier (LDCC), etc. SMD chip assembly includes ball grid array (BGA), chip size packaging (CSP), etc.
7.2.5 SMT Design
The SMT design should consider various system requirements such as functionality, power consumption, frequency range, power input conditions, etc. Before going for SMT layout, the design should also consider components, substrate, and process selections. The basic rules for SMT design are as follows:
1. Circuit Partition Principles
(1) It has a block design according to function.
(2) It has separate analog and digital circuits.
(3) It has separate , middle-, and low-frequency circuit design, when necessary high-frequency part should be shielded.
(4) It has separate high power circuits from other circuits, for better heat dissipation.
2. SMT Substrate Design Principles
Substrate can be designed in “puzzle” fashion.
(1) The “puzzle” can be assembled by the similar or different circuit boards.
(2) The maximal size is determined by a “pick and place” machine and reflow oven pa-rameters.
(3) A working area of 3–4 mm from the edge should be provided, and fiducial marks on the opposite corners should be designed.
(4) Each individual board is routed, which leaves the connecting bridge with the correct size and proper strength after V-grooving.
3. SMD Layout Principles
(1) The arrangement of SMD should follow its footprints; the same type devices should
130 Chapter 7 Module Assembly and Optoelectronic Packaging line up in the same direction for ease of placement and inspection. This is also optimal for automatic pick and place. One carefully designed PCB is shown in Figure 7.6.
(2) Components, axes should be parallel or perpendicular to each other.
(3) The distribution of components should be even and give plenty board area for power devices.
Same orientation as SOT23
Marker for IC orientation
Chip type components pitch and axis are parallel
Figure 7.6 A SMD schematic layout
4. Pad Layout Design Principles
There are various pad patterns. The pad design will affect the reliability of the soldering joints, and the datasheet will show the correct pad pattern and parameters. Here are the general rules for pad layout.
(1) The pad pitch should equal the corresponding lead pitch.
(2) The pad width should equal the corresponding lead width plus or minus the K factor, which is determined by the component tolerance and placement accuracy.
(3) The pad length is determined by the lead’s height and width and by the lead to pad soldering area. Pad length plays a greater role in solderability than the pad width.
7.2.6 SMT Testing
SMT testing techniques can be divided into noncontact testing and contact testing. The noncontact testing has evolved from visual inspection to automatic optical inspection (AOI) and automatic X-ray inspection (AXI). The contact testing can be further divided into online-testing and functional testing. The basic SMT testing methods include the following:
1. In-Circuit Tester (ICT)—The Most Common Electronic Testing Instrument
The traditional in-circuit tester uses the custom made bed-of-nail to contact with test points on the finished PCB and applies less than 1 volt voltage and less than 10 mA current to conduct isolated tests to determine various electrical parameters of onboard components, such as resistor, inductor, capacitor, diode, transistor, thyristor, field effect transistor, IC module, ASIC, etc., and failure modes such as missing component, misplacement, out of spec ification, soldering short, joint open, and related specific components and locations. The bed-of-nail in-circuit tester is fast and suitable for the single configuration mass production testing and costs less. However, with increasing PCB density and smaller SMT pitch, particularly the shorter R&D and production ramp up time coupled with the increasing variety PCB types, the bed-of-nail in-circuit test lags in the following aspects—long lead time to make bed-of-nail fixture, long trouble shooting time, expensive, and high-density SMT assembly can’t be tested. The flying probe was developed based on bed-of-nail with
7.2 Surface Mount Technology 131 probes replacing the pins. A typical flying probe station has four heads with eight probes on it in the x-y stage, and its minimal testing pitch is 0.2 mm. The probes touch the testing points according to the preprogrammed coordination and sequences and conduct the functional test of various components and its open and short conditions. Therefore, the flying probe station outperforms bed-of-nail in test accuracy, minimal pitch, and not requiring custom made fixtures, and testing programs can be generated by PCB CAD software. The disadvantage of the flying probe is the relatively slow test speed.
2. Functional Tester
ICT effectively searches various failure modes in the SMT assembly. However, it can’t evaluate the performance of the whole PCB system. The functional tester can determine if the system fullfils its design specifications. It treats each target unit as one functional block, and by inputting various signals and testing outputs, it follows the design requirements to ensure normal operation. One simple functional test is to measure output signals after powering up the PCB assembly along with its supporting circuits. If the circuit assembly works normally, it passes the test. This method is simple and costs little. However, the failure mode cannot be detected automatically.
3. AOI
With increased packaging density, the contact electronic testing is facing increasing chal-lenges, and the incorporation of AOI into online SMT testing is an important improvement in testing techniques. AOI is capable of inspecting not only soldering quality but also the qual-ity of light pipe, solder paste, and placements. The AOI can replace most human operations and increase both product quality and production efficiency. The AOI camera automatically scans various portions of the PCB, compares the solder images with data from the library, determines PCB defects by image processing, and marks the defects on the screen for trou-ble shooting. The modern AOI system has adopted advanced computer vision, new lighting schemes, increased magnification, and complex algorithms, which enables fast and accurate error detection. AOI detects the following errors–missing components, wrong polarity of tantalum capacitor, misplacement, tilt, bending, folding, excess or insufficient solder, solder bridging, cold solder, etc. In addition to its ability to determine defects beyond human vision, AOI also gathers the product quality and defect mode data in all manufacturing processes for further analysis. However, AOI can’t inspect electrical failures and invisible soldering points.
4. AXI
The AXI process is rather simple–after a PCB is fed into the machine, the X-ray is emitted by the X-ray tube above the PCB, goes through the board, and is detected by detectors underneath. Solder contains a large amount of lead, which absorbs ray. Therefore, the X-ray passing through the solder is much weaker than that of fiberglass, copper, or silica; thus the X-ray image for the solder is sharp, which makes it easy to find solder defects after simple imagine analysis. The AXI progressed from 2D to 3D inspection, the 2D version transmits X-ray inspection, which can generate a sharp image of the solder point on a single-sided PCB. However, for the double-sided SMT PCB, the solder joints on both sides will overlap and are difficult to separate. The 3D inspection uses tomography technology, which focuses the X-ray to the desired layer and projects the image to a rotating plane, where the image in the focal plane is sharp while images from other layers are blurry, thus enabling separate images of solder on each side. The 3D X-ray tomographic technology can not only inspect double-sided PCB, it is also capable of observing invisible solder, such as those in BGA, and give “sliced” images of BGA–solder’s top, middle, and bottom. In addition, it can
132 Chapter 7 Module Assembly and Optoelectronic Packaging also inspect through-hole soldering, such as the filling of a hole, thus greatly improving the soldering quality.
7.3 Packaging of Flat Panel Display Modules